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The water balance model adapted to the local conditions provided a useful tool for the management of recharge rates and thus GW resources. At field level, recharge rates are higher with deep GW levels and silt clay loam soil texture (D-SCL) than with shallow GW levels and silt loam soil texture (S-SL) and medium GW levels and silt loam soil texture (M-SL). The higher recharge under D-SCL is due to a smaller capillary rise contribution in this soil, which increases the net irrigation requirements and hence the recharge rates. For example, cotton has a 35 mm higher recharge under D-SCL than S- SL. In HRU-2 and HRU-3, with S-SL characteristics and larger share of cotton than all other HRUs, the highest recharge rate was 9.6 mm d-1 during the month of August. HRU-1 also has S-SL soils, but the maximum recharge during August was only 7.4 mm d-1. The low recharge rate in this HRU-1 compared to HRU-2 and HRU-3 is consistent with the lower share of cotton. This shows that recharge rates are also under the strong influence of land use. At WUA level, the maximum recharge rate is 7.2 mm d-1 during the month of August, which is quite high compared to values in other studies, e.g., on the Indus basin. In the developed scenarios, the monthly trend of the recharge rates is similar to the baseline scenario, i.e., maximum in August and lowest in April, May and September. However, the recharge reduces with the increase in irrigation efficiency. This relationship was well described by creating the exponential function (coefficient of determination near to 1). The equation of the exponential curve shows that recharge rates decrease with the improvement of irrigation efficiency and come closer to zero with a maximum irrigation efficiency ratio of 1. The relationship can be used for managerial purposes by the water managers in the area to control the recharge.

Abstract: Modeling groundwater recharge at field level and GIS-based approach to upscale from field to scheme level

Accurate quantification of the rate of groundwater (GW) recharge is a pre-requisite for the sustainable management of GW resources. This study presents (1) a method to determine recharge at field level and (2) a procedure for up-scaling from field to scheme level. The field level investigations can capture the complex processes, e.g., upward flow of water under shallow GW conditions, which have been disregarded worldwide for years in the past when estimating the recharge at a larger scale. For this purpose, a

water balance model at field level was modified to suit the local conditions of the Shomakhulum Water Users Association (WUA) in the southwest of the Khorezm region in Uzbekistan. Recharge in this water balance approach is considered as a fraction of the irrigation water losses, which can be determined by estimating difference between net and gross irrigation requirements. Net irrigation requirements were calculated by subtracting the capillary rise (using the HYDRUS-1D) and rainfall from the potential evapotranspiration (using the FAO CROPWAT model) for the crops grown in the area. Determination of gross irrigation requirements was based on the efficiency concept. Recharge at field level depends on several factors, e.g., climatic conditions, soil texture, cropping pattern and GW levels. These factors can vary significantly in any irrigation canal command area. To capture the spatial variability of these factors, Arc GIS was used. Thiessen polygons were drawn in Arc GIS to capture the spatial variability of GW levels and soil texture, which determine the capillary rise and in turn the water balance and hence the recharge. Satellite remote sensing was used for land-use classification. Based on the GW and soil texture maps, the six subunits of the WUA with homogenous GW conditions and soil textures are considered as hydrological response units (HRUs). HRU-1, 2 and 3 have shallow-silt loam (S-SL) characteristics, HRU-4 has deep-silt clay loam (D-SCL) and HRU-5 and 6 have medium-silt loam (M-SL) characteristics. Recharge calculated at field level was first up-scaled for these HRUs and then for the whole WUA. To quantify the impact of improved irrigation efficiency scenarios on recharge rates, 4 irrigation efficiency scenarios were developed: 1) current irrigation efficiency (S-A), 2) improved conveyance efficiency (S-B), 3) increased application efficiency (S-C), and 4) improved conveyance and application efficiency (S-D).

Results show that GIS provides a useful environment to capture the spatial variability of physical factors (GW and soil texture) that influence recharge. During the monitoring year 2007, land use was identified as the driving force. The area under cotton has the highest recharge (895 mm under D-SCL) in the peak irrigation period after rice (2514 mm), but a very low share of rice (1 %) in the considered WUA is the reason for the rather small influence of rice cultivation on the recharge in the WUA. Due to higher

(4.4 mm d-1), which shows that recharge is strongly influenced by the share of cotton. The high recharge rates in the cotton fields are not only due to the crop-water requirement but also due to special treatments the crop receives from the water management planners because of its strategic importance. For the different irrigation efficiency scenarios, it was found that by improving the irrigation efficiency, seasonal recharge reduced from maximum of 4 mm d-1 (S-A) to a minimum of 1.4 mm d-1 (S-D). The curve established for recharge and irrigation efficiency scenario can provide guidelines for the water planners in the region for regulating irrigation schemes keeping in mind the effect on recharge rates.